1. Field of the Invention
This invention relates to a novel microemulsion system and a method of recovering crude oil therewith. More specifically, this invention relates to a novel water-external microemulsion system comprising at least 90% by weight of an aqueous medium, an oil and as the surfactant, a C.sub.8-16 orthoxylene sulfonate and a method of recovering crude oil therewith through a secondary or tertiary microemulsion flooding technique.
2. Description of the Prior Art
The crude oil which is accumulated in subterranean formations is recovered or produced therefrom through one or more wells drilled into the subterranean formation with the initial production of the crude oil being carried out by what is referred to as "primary recovery," i.e. where only initial formation energy is used to recover the crude oil. The primary recovery technique, however, leaves substantial quantities of crude oil in the subterranean formation. Accordingly, the recognition of the large amount of crude oil in many oil-producing subterranean formations and reservoirs has led to the use of so-called "secondary and tertiary recovery" techniques which have as their primary purpose the economical recovery of additional quantities of oil known to be present in the subterranean formation or reservoir after primary depletion.
Probably one of the most common secondary recovery techniques is the so-called "waterflooding" in which aqueous fluids are injected at one or more points in the reservoir at pressures sufficient to be forced out into the reservoir and toward a spaced production well or wells so as to effect a displacement of the oil from the pores of the reservoir and a driving of the oil ahead of the water front to the production well or wells.
As can be easily understood the waterflooding technique and similar secondary recovery techniques are only advantageous when the cost of the water and any additional necessary chemical modifiers is less than the value of the oil which is recovered after primary depletion. Accordingly, the displacement efficiency of the waterflood and similar techniques has been a determining factor in connection with whether such techniques can be satisfactorily used in oil recovery.
The displacement efficiency of the water itself is relatively poor primarily due to the discontinuous oil droplets which are formed as the waterflood proceeds. There is a relatively high interfacial tension between the water and the oil which contributes to the capillary retention of the discontinuous oil and thereby prevents its displacement by water under pressure gradients feasible in reservoir flooding processes. The displacement efficiency decreases with increasing interfacial tension thereby making recovery of oil quite difficult.
As a result of the foregoing, various aqueous surfactant systems have been proposed for use in waterflooding processes for recovering oil. Thus, it has been proposed that the interfacial tension between the oil and water can be reduced from a characteristic value of the order of 35 dynes per centimeter to a value of less than 1.0 dynes per centimeter with the proper selection of a surfactant or surfactants. Here again, however, the selection of a particular surfactant depends not only upon the ability of the surfactant to reduce the interfacial tension between the oil and water but in addition the cost of the surfactant, since again in any secondary or tertiary recovery technique the cost of the materials utilized must be significantly less than the value of the oil which can be recovered.
Not only have surfactants been utilized in secondary recovery techniques such as waterflooding, but in addition, aqueous surfactant solutions have been employed in tertiary recovery techniques utilized to recover residual oil from a "watered-out" reservoir. In such uses a slug of an aqueous surfactant system will be introduced into the watered-out reservoir followed by a driving fluid to drive the slug of aqueous surfactant through the reservoir so as to allow displacement of the residual oil trapped in the pores of the reservoir.
More recently, secondary and tertiary recovery techniques have turned toward the applicability of microemulsions, most usually oil-external microemulsions comprising an oil, i.e. a refined or crude oil, an aqueous medium, sufficient surfactant to form the microemulsion, and optionally an electrolyte and one or more co-surfactants. Such microemulsions have advantages when compared with aqueous surfactant solutions. One such advantage is that in the microemulsion there is a lessened tendency for the surfactant to be adsorbed on the formation rocks, thereby allowing a more efficient utilization of the surfactant for displacement of the crude oil in the secondary and tertiary recovery techniques.
With regard to the displacement efficiencies of microemulsions, it has been determined that the single-phase transparent or translucent microemulsions miscibly displace the crude oil when the microemulsion is in the single-phase state, but when the microemulsion breaks down into a multi-phase system, the displacement of the crude oil becomes immiscible. Accordingly, when considering microemulsions from the standpoint of a ternary diagram of oil, water and surfactant, increased efficiency can be associated with a minimization of the multi-phase region with an associated increase in the single-phase region and low interfacial tensions in the multi-phase region which are reached upon breakdown of the single-phase microemulsion upon dilution with the formation crude and water on the front edge and driving fluid on the back edge. Accordingly, these are the criteria which are used in developing efficient microemulsion systems.
Microemulsions have been proposed which contain a substantial quantity of water, and which are generally water-external microemulsions. The use of water-external microemulsions are generally advantageous from the standpoint of cost since a relatively small quantity of oil is used to form the microemulsion. The primary disadvantage, however, is that such microemulsions often do not provide decreased surfactant adsorption and minimum interfacial tensions in the multi-phase region of the oil-water-surfactant ternary diagram. Accordingly, there is a continuing need for an effective microemulsion system which can eliminate these deficiencies.